You are forgetting about armature inertia.
Granted, window regulator / track friction can be high enough to limit travel so we are not talking about inches of travel.

Why do you feel that 12 V+ is supplied to both terminals of the motor when no movement is requested?

Hopefully Turbo has or will soon just trouble shoot his window electrics. This thread has gone off track with very unlikely scenarios.

Check fuses, relays, motor, switches, wiring using meter, 12v supply and wiring diagram. Many here have done that.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Not sure about the 240D, but on my 300D, the power supply to the switches/motors is connected in pairs. Front left & Rear right and vice versa. So if passenger rear works and driver front does not, at least you know that power is being supplied. Moves trouble shooting to next step.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Your theory is correct, but it’s not relevant to this application. The window motor produces a tiny fraction of a horsepower, turns at maybe 500 rpm, and drives a complex mechanism with high friction loads. There isn’t enough residual motion to provide much generated power, so essentially no braking force. The problem you describe is one of hand/eye coordination.

If the mechanism stops due to friction braking, so does the armature. When there’s no motion, no current is generated. All the mechanical energy is dissipated as heat.

As for 12v+ on both leads, it’s the way every Benz is wired. It reduces by one the number of conductors over the next simplest scheme. It also snubs the inductive kick which occurs when the motor stops dead due to mechanical braking.

Just to be sure 240D is same as my 300D, checked 240D wiring diagram. As can be seen below, Fuse Fb feed both sides of motor when switch is in neutral position.

Meter across motor terminals should read ~zeroV with switch in neutral. Then +12V or -12V when switch is actuated up/down. Same if wires are first disconnected from motor.

If you do see +/- 12V and motor doesn't run, take motor out. Replave or open it up and check the thermal overload fuse. Then test motor while out with fuse jumpered. Some have left fuse jumpered and re-installed. If you do that be careful not to hold switch down too long!

Attached Thumbnails

 

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Can someone tell me how many gear teeth on the rear window motor in a coupe (1983 300CD)?

Okay so I fixed my windows, broken wires in the doors, but now I can't get my hood open I already tried manually opening the latch through the hood. The drivers side released but the passenger side won't give. Any ideas on how to get it open.

Sent from my Pixel 2 using Tapatalk

On a 97 E320 / W210 I've used a long screwdriver ( like 3 ft long ) to pop a latch from under the car.

I am just going to throw this in. I don't like to use incandescent test lights because when the time comes to use one the bulb is often shot and it is often difficult to find good ground for it near where you are using the light. I prefer a volt meter because it tells me how many volts I am getting. I have found that to be of the most use most of the time.

I admit I had a Car battery fail and it showed 12 Volts but had so little current that it barley would spark if you shorted a cross the Battery Terminals and that is how I found out it was no good.
In that particular case a working test light would have helped.

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84 300D, 82 Volvo 244Gl Diesel

What exactly did you do to fix the windows?

All 4 Doors had broken wires?

There is some threads on the hood not opening issue I remember none of the details. It has been several years since I have seen someone post with that problem.

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84 300D, 82 Volvo 244Gl Diesel

Using a volt meter on an electronic circuit board fed by a well regulated power supply will work. However , a car has a unregulated supply and long wire runs. This makes for " yep, the voltage is slightly low but is it due to a fault or normal resistance? " .

Some thing amiss here as there's only _one_ hood latch on a 1982 W123 so how can only one side of the hood be opening / .

-Nate

My knowledge is only from actual experience. I have no theory or other electronics knowledge.

I mean if I was checking + wire to the Window and I had 12 volts instead of 12.50 volts due to wire resistance I would deem that enough voltage ao allow the window to work.

However, I recently had a problem on my Chevy Van Window. In this case there was some but not all of the wire strands broken (about 1/3 of the strands broken on 2 different wires) and that apparently caused enough resistance in the Window Switch and I eventually had smoke coming out of the Switch. The Window only worked intermittently and finally not at all.

Since the broken strands were near the connectors I ran a bead of Solder on each wire to the connector allowing a good circuit and an New Switch and that fixed it.

In that case although it was trouble shot without an Meter or Test Light I would have been showing good voltage and it is likely the Test Light would have also worked fine as the Test Light would not need a lot of current.

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84 300D, 82 Volvo 244Gl Diesel